High performance heat transfer tube and method of making same

ABSTRACT

A high performance heat transfer tube for use in the condenser for an air conditioning or refrigeration system is formed on a grooved mandrel which has a sufficient number of grooves i.e., thirty six to forty eight, with a suitable helix angle, such as thirty degrees, and a sufficiently small pitch, such as 0.10 inches or less, so that a Moire imprint is avoided in the external fin enhancement. This makes the fin height uniform over the exterior of the tube, which results in an increase in condenser efficiency.

This is a division of application Ser. No. 111,917, filed Oct. 21, 1987,now U.S. Pat. No. 4,866,830.

BACKGROUND OF THE INVENTION

The present invention relates to heat exchangers, and is moreparticularly directed to heat exchangers which have tubes fortransferring heat between a coolant liquid flowing through the tubes anda refrigerant fluid in contact with the exterior of the tubes. Thepresent invention is more specifically directed towards tubes which havean internal rib enhancement and an external fin enhancement, and alsotowards an improved method for making such tubing.

In the condenser portion of certain refrigeration or air conditioningsystems, a coolant fluid, such as water, is passed through heat transfertubing while refrigerant vapor in contact with the exterior of thetubing changes state from vapor to liquid, giving up heat ofcondensation to the coolant liquid within the tubing. The external andinternal configuration of the tubing is important in determining theoverall heat transfer characteristics of the tubing, and hence indetermining the efficiency of the system. With condenser tubing that hasan internal rib enhancement and an external fin enhancement, thecondensation activity takes place at the tips or extrema of the fin, andthe condensate flows into the channels between the fins. The condensedliquid refrigerant fills the channels to a point at which the coolantdrips out. An internal enhancement, in the form of spiral or helicalribs or fins, causes a swirling of the flowing coolant within the tube.This induces some turbulence, which breaks up laminar flow and thus alsoprevents any insulating barrier layer from forming at the inner wall ofthe tube.

Tubes that are given both an internal and external enhancement aredescribed, for example, in the commonly-assigned U.S Pat. No. 4,425,696.Although that patent is directed to an evaporator, rather than acondenser tube configuration, a heat transfer tube suitable for use as acondenser tube could be constructed on the same tube finning machine,omitting the step of rolling the fins that is described in that patent.Other finned tubes for heat transfer are described in U.S. Pat. Nos.4,059,147 and 4,438,807.

In the tube finning machine employed in the production of this tubing, acylindrical grooved mandrel within the tube produces the internal rib,while a tool gang of discs carried on a tool arbor produces a finconvolution on the exterior of the tubing. The force of the gang ofdiscs on the metal tubing and against the mandrel causes the metal ofthe tubing to flow up between the discs to form the fins and down intothe mandrel grooves to form the ribs. At the locations of the grooves,however, there is less force placed on the metal, and the tubing metaldoes not flow as far outward between the discs of the tool gang. As aresult, there is a reduced height in the external fin at locations whichcorrespond to crossings of the fins with the internal rib. This producesa visually noticeable Moire pattern in the fins. Generally, the externalfin has a height of about 0.030 inches, but the extent of dip orshortening due to this Moire imprint is about 0.005 to 0.008 inches.

As aforementioned, in a condenser tube the tips or extrema of the fin iswhere most of the condensation activity takes place. However, because ofthe significant Moire reduction in height, where the fin crosses thepath of the rib the amount of exposed fin is significantly reduced. Thereduction in efficiency of condensation of refrigerant can exceedtwenty-eight percent, as compared to a finned tube where the fin heightis uniform over the circumference of the tube.

A way to produce condenser tubes with a uniform external fin height withan internal enhancement has long been sought, but no one has previouslybeen able to produce such a tube.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a heattransfer tube having superior efficiency characteristics when employedas a condenser tube.

Another object of the present invention is to provide an efficientmethod for making high performance heat transfer tubes for use ascondenser tubes in a refrigeration or air conditioning system.

More specifically, it is an object of this invention to provide fin- andrib-enhanced tubing, and a method of making same, which avoids the Moireimprint on the external fins.

In accordance with an aspect of this invention, a heat transfer tube isproduced with a plurality of helically extending interior ribs and atleast one helically extending fin, with the fin defining open channelsin which the condensed refrigerant coolant can collect. According tothis invention, the interior ribs are disposed at sufficiently smallpitch, and with a suitable helix angle, so that the exterior fin isformed without a Moire reduction in height at the positions where theexterior fins cross the interior ribs, and so that the distance frombase to tip of the fin is substantially uniform. Preferably there are 36to 48 of said internal ribs taken around the internal circumference ofthe tube, and the helix angle of the internal rib is on the order ofabout 30 degrees. This tubing is made employing a mandrel that has about36 to 48 helical grooves thereon which are cut with a helix angle ofsubstantially 30 degrees. The mandrel grooves have a pitch on the orderof 0.10 inches or less, and in a preferred embodiment of 0.070 inches.

The use of a mandrel having a high number of internal fins with a smallpitch results in a decrease in the Moire imprint. When the number ofgrooves on the mandrel was increased from the now-standard fourteengrooves (with a 45 degree helix) to thirty-six grooves or forty-eight(with a 30 degree helix) the Moire imprint was reduced and virtuallyeliminated in the case of the forty-eight groove mandrel. The reductionin Moire imprint was accompanied by an increase in both the refrigerantside performance, approaching that of a smooth internal finned tube, andin overall tube performance. That is, by using more grooves and reducingthe helix angle, an increase in performance was obtained on the coolantside. Even though there were more grooves than in previous attempts,there was no sacrifice in pressure drop performance on the water orcoolant side of the tube because of the corresponding reduction in helixangle.

It is thought that the internally ribbed tubes with helical fins, withtheir characteristic Moire imprint, have a wider finned tip in thedepressed region of the Moire, and this affects the condensate filmthickness, and liquid drainage characteristic in that area. This, inturn, results in lowering the condensate efficiency. That is, byreducing the Moire imprint effect on the fins, there will be a highercondensing coefficient. Previously, such an elimination or reduction inthe Moire imprint was achievable only by producing a smooth orunenhanced inside surface of the tube. However, this reduced thewater-side or coolant-side efficiency and limited the overallperformance of the tube. Also, if the helix angle of the internal finwere selected to be high to correspond with the helix angle of theexternal fins, the water-side or coolant-side pressure drop would becometoo great, and efficiency would actually drop. However, with the tubeenhancement according to this invention, the Moire imprint issubstantially eliminated, while maintaining optimum coolant-sidepressure drop and heat transfer characteristics.

The above and many other objects, features, and advantages of thisinvention will be more fully understood from the ensuing description ofa preferred embodiment, which should be read in connection with theaccompanying Drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic sectional view of a condenser tube in the processof production, a grooved mandrel, and a tool arbor with tool gang forrolling a tube on the grooved mandrel to form the helically finned andribbed heat transfer tube according to this invention.

FIG. 2 is an enlarged sectional view of the tube wall of the heattransfer tube with fin and rib enhancements according to this invention.

FIG. 3 is an enlarged sectional view of a heat transfer tube of theprior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention as described below has designedespecially for use in a condenser of a refrigeration or air conditioningsystem of the type in which a coolant liquid, which can be water, passesthrough the interior of the heat transfer tubes, and in which arefrigerant is condensed from vapor form to liquid form in contact withthe external surfaces of the tubes. Typically, there are a multiplicityof these heat transfer tubes mounted in parallel and connected so thatseveral tubes form a fluid flow circuit and there are several of suchparallel circuits provided to form a tube bundle. Usually, all of thetubes of the various fluid flow circuits are contained within a singlecasing that also contains the refrigerant in the form of a condensedvapor or gas. The heat transfer characteristics of the condenser arelargely determined by the heat transfer characteristics of theindividual condenser tubes.

Referring now to the drawing, and initially, to FIG. 1 thereof, a tubefinning machine is shown in elevational cross section, and this machinecomprises a tool arbor 10 with a tool gang 12 formed of a plurality ofdiscs 14. At the axial position of the tool gang 12, there is disposed amandrel 16 mounted on a mandrel shaft 18. The mandrel has a number ofgrooves 20 cut therein which correspond to the pattern of ribs that areto be formed in the tube. In this case, the mandrel 16 has forty-eightgrooves 20, as opposed to the fourteen grooves that are found on themandrel that is used in conventional enhanced tube manufacture. Thesehelical grooves 20 have a helix angle of about thirty degrees, and areat a pitch or spacing of 0.070 inches.

A tubular workpiece 22 in this embodiment is a copper blank tube of 3/4inch nominal outside diameter. The workpiece is supported on the mandrel16 beneath the tool gang 12, and the discs 14 on the arbor 10 arebrought into contact with the tubular workpiece at a small anglerelative to the longitudinal axis of the workpiece. This small amount ofskew provides for a longitudinal driving of the workpiece 22 as thearbor 10 is rotated. The discs 14 displace the copper material of thetube wall, causing the material to flow downward into the grooves 20 toform an internal rib enhancement 24 and to flow up between the discs 14to form an external fin convolution 26. As shown in more detail in FIG.2, the fin structure 26 generally has a base 28 towards the axis of thetube and in contact with the tube wall, and a tip 30 remote from thetube wall. The base 28 is somewhat wider, axially, than the tip 30.Channels 32 are defined by spaces between the fins, and serve aslocations for the condensed refrigerant to collect.

As aforementioned, the height of the fin, that is, the base-to-tipspacing, should be uniform everywhere along the circumference of thetube 22. The fin 26 also has a profile that is uniform over thecircumference of the tube 22. This is achieved with the internal ribenhancement having the number of helical ribs, pitch, and helix angleaccording to this invention.

As shown in FIG. 3 for comparison purposes, in the condenser tube of theprior art, in a condenser tube 22' of the prior art, the internal ribenhancement 24' has a greater pitch or spacing between the internalribs, and as a consequence in the external fin enhancement 26', there isa dip 34 or shortening of the fin at the crossings of the fin 26' with arib 24'. This shortening or Moire results in a non-uniformity of aboutthree to eight mils, and limits the exposure of the fin enhancement 26'that is available for condensing the refrigerant.

While the present invention has been described with respect to apreferred embodiment, it should be recognized that many modificationsand variations would be apparent to those of skill in the art withoutdeparting from the main principles of this invention. It should berecognized, for example, that for tubing made of a different material,or with a different diameter or tube wall thickness, a mandrel 16 havinga different number of helical grooves 20 or having the grooves 20 at adifferent helix angle or with a different pitch, might be employed.Also, while the preferred embodiment described here relates to acondenser tube, the same principles could readily be transferred to theproduction of an evaporator tube. Accordingly, it should be understoodthat many other embodiments of the present invention may be made withoutdeparting from the scope and spirit of this invention as describedherein and as defined in the appended claims.

What is claimed is:
 1. In a heat transfer tube for transferring heat ofcondensation of a refrigerant fluid outside the tube to a circulatingcoolant liquid inside the tube and which is enhanced with a plurality ofhelically extending interior ribs on the interior surface of the tubefor inducing turbulence in the circulating coolant liquid and at leastone helically extending exterior fin about the exterior of the tubeextending radially outward from a base portion disposed toward the tubeaxis to a tip portion radially remote from said axis, the fin definingat least one open channel in which the condensed refrigerant fluid cancollect; the improvement wherein said external fin has a substantiallyuniform profile over the circumference of the tube, and said interiorribs are disposed at sufficiently small pitch that the exterior fin isformed without a Moire reduction in height at the positions where theexterior fin crosses the interior ribs and the distance from the base totip of said fin is substantially uniform.
 2. The heat transfer tubeaccording to claim 1 in which there are at least 36 of said internalribs.
 3. The heat transfer tube according to claim 2 in which said ribshave a helix angle on the order of 30 degrees.
 4. The heat transfer tubeaccording to claim 1 in which the internal ribs have a pitch on theorder of 0.10 inches or less.
 5. The heat transfer tube according toclaim 1 in which there are 48 internal ribs with a helix angle of about30 degrees.